The storm struck suddenly around 3 p.m. Saturday as the market was in full swing.
Dawn Taylor, who was on scene, said people scattered as the sky darkened and heavy rain began to fall.
“Everyone ran inside. The wind was blowing doors open,” she said.
The storm was over just as quickly as it had arrived, but the outdoor market was in chaos.
“The vendors' tents were ripped apart, food everywhere, dirt and mud all over the buildings,” Taylor said. “People were scared, very scared.”
Waterloo Region EMS said five people were injured. One was transported to hospital with non-life-threatening injuries; the others were treated on scene.
A severe thunderstorm watch was in place for the area at the time, but no warnings had been issued prior to the storm.
Rob Kuhn, a severe weather meteorologist with the Ontario Storm Prediction Centre in Toronto, said the damage was caused by straight-line winds, not a tornado.
“I suspect from the radar data that they may have experienced a wind gust approaching 100 km/h,” he said.
The same cell caused widespread damage in nearby Kitchener-Waterloo. Winds snapped large branches off trees, pulled up fences, toppled signposts and damaged roofs.
Kuhn said Doppler radar indicated the cell was producing wind gusts up to 104 km/h at the time it was moving over the south end of Kitchener.
“I was watching the webcam at the airport when it came in and it looked like a classic gust front,” he said.
More thunderstorms rumbled through southern Ontario Saturday night, some of them packing even stronger winds than the afternoon storms. A peak wind gust of 100 km/h was recorded in Essex shortly after 9 p.m.
Southern Ontario has been baking through a late summer warm spell, and a cold front sweeping through the region created favourable conditions for the development of severe thunderstorms.
On Friday morning, severe storms ripped through northern Ontario, even prompting a tornado warning.